910

SUGAR. —Vacuum Pan.

some “ loss of colour ” is produced. Pans withrather pointed constricted bottoms are to be pre-ferred when it is desired to “form grain” in a small“ charge,” with the view of building the compara-tively small numbers of crystals so formed in thefirst instance up to the maximum size, in one fillingof the pan.

Steam and Water required to Work a Vacuum Pan.—Let it be supposed that 30,000 lbs. (about 13-5

WJHH.QU1CK.

tons) of green sugar (masse - cuite ), containing 10per cent, of water, be boiled out of liquor of 50 percent, sugar contents (27£ 0 B.) in two hours: then30,000 — 3000 = 27,000 lbs. sugar is in that masse-cuite; but that sugar was dissolved originally in itsown weight of water, or 27,000 lbs., of which 3000lbs. remain unevaporated, or 24,000 lbs. of waterhave been converted into steam in the two hours.Assume that two-thirds of this quantity is evapor-ated in the first hour, while the contents of the panare comparatively “thin,” then 16,000 lbs. of watervapour have to pass through the neck of the panand be condensed in one hour. If the pressure inthe pan be equal to 2 inches of mercury (“ a 28-inch

30

vacuum ”), the volume of this vapour will be =

15 times as great as at the atmospheric pressure.According to Kegnault’s experiments, saturatedwater vapour of this tension weighs 0-003 lbs. percubic foot; and leaving out of sight minute correc-tions, we may take it that under the conditions given,

the 16,000 lbs. of water will form

16,000 X 1-003

= 5,333,334 cubic feet of vapour to pass throughthe neck in 60 X 60 seconds, or 1481-5 cubic feetper second; and if the neck have an area of 2 square

feet, the velocity of the vapour must be

1481-5

2

740-75 feet per second. With a wider neck, thevelocity will be proportionately less, and vice versa,

and the resistance caused by friction will vary atthe same time; but in what proportion or degreethere is, as far as I know, nothing determined. Inpractice wide and short necks are found to facilitatethe maintenance of a good vacuum.

Condensing Water Required. —In the case supposed24,000 lbs. of water were evaporated in two hours,and were converted into vapour of one-fifteenth ofthe atmospheric pressure. Now, as the minimumpressure obtainable in any apparatus containingwater is that due to tension of the vapour in thecoolest part of that apparatus, the air-pump mustnot have a temperature exceeding 101° Fahr. intheory, and in practice to get such a vacuum notmore than 80° Fahr. The colder the pump is kept,supposing the whole apparatus to be tight, thebetter will be the vacuum, and consequently thecolder will the pan boil.

Let us assume, for the sake of simplicity, that allcondensing water of the initial temperature of 60°Fahr. runs away at an average of 90° Fahr., andthat the vapour from the pan is saturated steam(it is actually superheated), 1 lb. vapour of a tensionequal to 2 inches of mercury contains 1145T unitsof heat. The pound of water resulting from itscondensation at 90° Fahr. contains 90 of those units,leaving 1055 units to be taken up by the condensingwater. Each pound of water heated from 60° to

1055

90° Fahr..absorbs 30 units of heat; so — 35'2

uU

lbs. of w r ater at 60° Fahr. are required to condense